1. Field of the Invention
This invention is in the field of computer-generated raster graphic systems and, more particularly, relates to improved methods and apparatus for displaying a cursor having any one of a plurality of possible forms in any position on the raster.
2. Description of the Prior Art
Raster scan CRT displays form a principal communication link between computer users and their hardware/software systems. The basic display device for computer-generated raster graphics is the CRT monitor which is closely related to a standard television receiver. To achieve the full potential of raster graphic systems, such systems require digital computational support substantially in excess of that provided by the typical CRT monitor. The development of large-scale integrated circuits and microcomputers makes it possible to control such displays at affordable prices. Typically, each picture element (pixel) of a substantially rectangular array of such elements of a CRT comprising the raster is assigned a unique address, which address is comprised of the x and y coordinates of each pixel in the array. Information to control the display of a pixel, its color and intensity, pixel control information, is stored in a random-access pixel memory at a location having an address corresponding to that of the pixel. The source of such pixel control information is typically a microcomputer located in a graphic controller. Such pixel control information may include the address in a color look-up memory at which location there is stored binary control signals which are used to control the intensity and color of each pixel of the array as it is scanned.
The horizontal and vertical sweep signals of the raster scan are digitized to produce the addresses of the pixels as scanned. The binary address signals produced by the raster scan logic of the system are applied to the pixel memory into which the graphic controller has previously written the pixel control signals, or information, determinative of its display. The pixel memory produces such signals in synchronization with the scanning of the addressed pixel by the CRT monitor by the system. The pixel control signals stored in the addressed location of the pixel memory are an address in the color look-up memory and are applied to the color look-up memory. Digital color control signals stored in addressable locations of the color look-up memory are read out of the addressed locations of the color look-up memory in response thereto. The digital color control signals are converted to analog signals by digital-to-analog (D/A) converters, and the analog signals produced by the converters are applied to the three color guns of the typical CRT monitor to control the intensity and color of each pixel of the raster as it is scanned.
Most raster graphic systems have the capability of displaying a cursor, a movable marker, in the raster. Prior art graphic systems known to applicants produce a cursor by programming the graphic controller to write into the pixel memory at pixel addresses within the boundaries of the cursor, pixel control signals which cause selected pixels within the boundaries of the cursor to display the cursor color at the cursor intensity; thus forming the cursor. Such prior art raster graphic systems typically limit the positions in which a cursor can be displayed to a character cell of 8.times.12 pixels, for example.
Writing pixel color information into a pixel random-access memory to cause a cursor to be displayed in the raster significantly increases the amount of communication between the graphic controller of the system and the pixel memory.
The problem is not only that of writing into the pixel random access memory at addressable memory locations which correspond to the raster addresses of the selected cursor pixel positions, control signals that will cause the pixels forming a raster to be displayed in the cursor color and intensity, but to store elsewhere, any pixel control signals stored in those addressable locations when cursor control signals are written into them, or any pixel control signals that may be subsequently written into those pixel memory locations by the system graphic controller until such time as the cursor is moved to a new position in the raster or turned off. When a cursor moves, or is turned off, the most recent pixel control signals stored elsewhere must then be written into their proper locations in the pixel memory. Providing the capability of displaying a cursor in prior art systems also increases the complexity of programming the graphic controller.
Thus, there is a need in color raster graphic systems to provide the capability of producing a cursor which can assume any of a plurality of forms without the need for the graphic controller storing and updating as needed the pixel control information stored at the locations into which cursor pixel control information is to be written and then restoring such pixel color information when the cursor is moved or no longer displayed. There is also a need to provide such systems with the capability of positioning the cursor with respect to any pixel of the raster, and of providing a method and apparatus for producing a cursor that functions properly in a system which has an interlaced vertical scan.